Thermo Sudoku
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How to play Thermo Sudoku
Standard Sudoku rules apply. Extra rule: digits along each grey thermometer must strictly increase from the bulb (circle end) to the tip. A 3-cell thermo could be 2,5,8 or 1,4,9 but not 3,3,7 or 5,4,8.
Full guide →
Digits must strictly increase along thermometer shapes
Easy
Thermo Sudoku Easy
Medium
Thermo Sudoku Medium
Hard
Thermo Sudoku Hard
Expert
Thermo Sudoku Expert
What is Thermo Sudoku?
Thermo Sudoku overlays thermometer shapes on the grid. Each thermometer has a bulb (circle) at one end and a tip at the other. The digits along any thermometer must strictly increase from the bulb to the tip. Combined with standard Sudoku constraints, this creates powerful chains of deductions — knowing the bulb is at least 1 and the tip is at most 9, you can calculate the range of valid values for each cell on the thermometer based on its position.
At a Glance
| Constraint type | Line Constraints |
| Typical givens | 20–28 |
| Difficulty rating | ★★★★☆ 4/5 |
| Avg. solve time — Easy | 10 min |
| Avg. solve time — Medium | 22 min |
| Avg. solve time — Hard | 45 min |
| Avg. solve time — Expert | 75 min |
How to Solve Thermo Sudoku
| Technique | What it does | Level |
|---|---|---|
| Range Restriction from Length | A thermometer with N cells must use N strictly increasing distinct values. The bulb is at least 1 and the tip is at most 9, giving a calculable valid range for each cell. | Beginner |
| Minimum/Maximum Bounding | The bulb value is ≤ 9 − (N − 1). The tip value is ≥ N. Apply these bounds immediately to prune all thermometer candidates. | Beginner |
| Overlapping Thermometer Deduction | Two thermometers sharing a cell give tighter combined bounds than either alone. | Intermediate |
| Forcing Chains via Thermometers | When a value is forced into a thermometer cell, all cells closer to the bulb must be smaller and all closer to the tip must be larger. | Advanced |
| Short vs. Long Thermometers | A 2-cell thermometer is simply a greater-than relationship. A 9-cell thermometer forces exactly 1–9 in order. | Intermediate |
Average Solve Times
Easy
10 min
Medium
22 min
Hard
45 min
Expert
75 min
Frequently Asked Questions
What is Thermo Sudoku?
Thermo Sudoku is a variant where thermometer shapes are drawn on the grid. Each thermometer has a bulb (rounded end) and extends through several cells toward a tip. The digits must strictly increase from the bulb to the tip — so every cell along a thermometer must hold a larger digit than the one before it.
How do I read the thermometer clues?
The bulb is the starting point and must hold the smallest digit on that thermometer. Each subsequent cell must be strictly larger. For example, a 3-cell thermometer starting at the bulb means the bulb holds at most 7 (since two larger digits must follow). A 5-cell thermometer means the bulb holds at most 5.
Can digits repeat on a thermometer?
No. Because digits must strictly increase, no digit can appear twice on the same thermometer. A 4-cell thermometer uses four different increasing digits. Combined with standard Sudoku uniqueness rules, thermometers create very tight constraints on every cell they pass through.
Is Thermo Sudoku hard to learn?
The core rule is simple — digits increase from bulb to tip. Beginners can start solving after understanding just that one constraint. What makes higher difficulties challenging is tracking minimum and maximum values for each thermometer cell simultaneously across many overlapping thermometers.
What is the best first move in Thermo Sudoku?
Look for long thermometers. A 7-cell thermometer can only start at 1 and end at 7, 8, or 9 — drastically limiting all seven cells. Also check for thermometers where the bulb or tip overlaps with a constrained row, column, or box, as this often pins the starting digit immediately.
How long does Thermo Sudoku take to solve?
Easy puzzles take 8–15 minutes. Medium puzzles typically run 15–30 minutes. Hard puzzles average 35–60 minutes and expert puzzles can exceed 90 minutes. The difficulty scales sharply as thermometer lengths decrease and the number of simultaneously active constraints increases.